Cellulose pulp, which is used for many purposes, usually requires bleaching for the removal of any colored substance therefrom, except when used for purposes not calling for its whiteness, as when it is used for a packing material, such as the core of corrugated cardboard.
It has hitherto been usual to use chlorine-containing bleaching agents, such as chlorine, sodium hypochlorite, sodium chlorite, and a bleaching powder, for bleaching such pulp, simply because they are inexpensive and high in reactivity.
These chlorine-containing bleaching agents exhibit an excellent bleaching action, but their reaction with lignin produces harmful substances including organic chlorine compounds, such as dioxin, as by-products, and thereby presents the problem of environmental pollution. Under these circumstances, it has been a matter of great importance for mankind to have the pulp manufacturing industry decrease, or eventually end the use of chlorine.
It has been becoming common practice on a worldwide basis to employ oxygen gas, or hydrogen peroxide as a bleaching agent in a part of a process for bleaching pulp in order to decrease the use of chlorine. It has, however, been impossible to realize a satisfactory reduction in the use of chlorine, since it has generally been impossible to bleach pulp satisfactorily with oxygen, etc. Attempts have been made to employ high-pressure oxygen gas at a high temperature to obtain an improved bleach of pulp by bleaching with oxygen, while decreasing the use of chlorine even by a small amount, but have presented a new problem, since the depolymerization of cellulose is promoted, and results in pulp of lower strength.
In order to solve the above problem, the applicant of this application has previously proposed a method of realizing an improved bleach of pulp by adding a bleaching assistant during its bleaching with oxygen, hydrogen peroxide, etc. (Japanese Patent Application Laid-Open No. Hei 5-186987).
The above method has been able to realize an improved bleach of pulp as compared with any prior method of bleaching with oxygen, etc., but has still been unsatisfactory, and unable to realize a satisfactory reduction in the use of chlorine. Moreover, the heavy foaming of a bleached pulp slurry has presented a problem of operation during the step of dewatering, etc.
We, the inventors of this invention, have made a serious study of a problem as to how it will be possible to achieve an improved bleach of cellulose pulp and thereby a greater reduction in the use of any chlorine-containing bleaching agent by employing oxygen or peroxide, while having cellulose retain its strength, and without having any problem of operation caused by the foaming of a bleached pulp slurry, and as a result, we have made this invention by finding that the use of a specific compound as a bleaching assistant provides a very effective and economical solution to the above problems.